Devices of performing certain function by introducing wave onto a disordered medium are being widely used. For example, in case of skin therapy using light of laser, the devices are widely used for removing abnormal cells in the human skin which is a disordered medium and also used for inspecting the inside of metal or buildings by means of waves.
Generally, when wave is incident on the disordered medium, multiple scattering is generated in the medium and thus intensity of wave decreases as the penetration depth of incident wave in the medium increases.
The decrease of intensity of wave according to the increase of penetration depth of wave causes bad effects on the actual application using wave. For example, for the removal of abnormal cells in the skin, laser with a certain intensity having must penetrate into the depth where abnormal cells exist. Considering the decrease of intensity, if intensity of laser is increased, skin not to be removed which extends to the abnormal cell may be damaged. Therefore, techniques for enhancing transmission characteristics of wave are required.
Regarding the enhancement of transmission characteristics of wave, it is proved that when wavefront of incident wave is adjusted, the reduction of reflection on the disordered medium allows the increase of the penetrating depth of wave in the disordered medium.
According to the above theory, wavefront of incident wave for the above effect varies according to the disordered medium and it is an issue to find such wavefront. As of now, regarding a method of finding wavefront of incident wave, a single point optimization method and a method using scattering matrix measurement are known.
According to the single point optimization method, wavefront of incident waves is controlled such that incident waves physically orthogonal to each other are simultaneously introduced into the disordered medium and then, waves which emitted from the penetrating surface of the disordered medium has constructive interference at one point. Since the single point optimization method increases the total intensity of penetrating waves by increasing intensity of wave at one point, it is disadvantageous that the increase of intensity of penetrating wave is not substantial. Further, in the single point optimization method, disadvantageously, intensity of wave at the penetrating surface must be measured to increase the penetrating depth of wave, i.e., to increase intensity of penetrating wave.
Meanwhile, in the method using scattering matrix measurement, when waves physically orthogonal to each other are introduced into the disordered medium, amplitude and phase of scattering wave by the disordered medium are measured and is made in the form of a matrix, and then wavefront of wave whose transmittance is increased is found by analyzing the matrix.
The method of scattering matrix measurement is divided into a method of using a transmission matrix and a method of using a reflection matrix. In case of the method of using a transmission matrix, amplitude and phase of waves emitting from penetrating surface of the disordered medium are measured two-dimensionally to obtain a transmission matrix, like the single point optimization method. Meanwhile, in case of a method of using a reflection matrix, amplitude and phase of waves reflected by the disordered medium are measured two-dimensionally to obtain a transmission matrix and then, the transmittance is increased by finding wavefront of incident wave having the minimum reflection.
It is advantageous that the method of scattering matrix measurement has higher increase of penetrating wave that the single point optimization method and can also use selectively transmission wave and reflective wave.
The method of scattering matrix measurement is divided into a method of using a transmission matrix and a method of using a reflection matrix. However, to obtain a matrix, information of amplitude and phase of wave must be obtained at the penetrating surface of scattering wave and the measurement of both amplitude and phase of wave at many points inside the surface is very difficult and it costs a lot.
Also, besides the disadvantages or problems of the measurement, the above two methods have problems of difficulty to apply the methods to the actual devices such as a laser treatment equipment. Skin has different transmission characteristics depending on people and even for the same people, depending on portions of skin. Therefore, every time treatment is made on a certain portion of skin of a certain person, an adequate wavefront of wave for the corresponding skin must be measured. For the single point optimization method and the method of scattering matrix measurement using transmission matrix, sensor must be embedded in the skin for the measurement and thus, it is actually difficult to apply the method.
Even in a method of scattering matrix measurement using reflection matrix, as described above, since both amplitude and phase of two-dimensional wave at the reflection surface in the disordered medium must be measured, the measurement is difficult and actually, it is difficult to apply the method to laser treatment equipment requiring a real-time measurement.